1. Field of the Invention
The present invention relates to an image-processing, and, in particular, to an image processing for compression and decompression of image data in a field of a digital camera, the Internet, images for medical treatments, and images handled by satellite communications, and so forth.
2. Description of the Related Art
Along with the progress in image input technology and image output technology, a demand for achieving highly definite images has increased in recent years. For example, when, as image input apparatuses, using digital cameras as an example, cost reduction has been achieved on highly efficient charge-coupled devices (CCD) on the order of three million pixels, and thus, such high-definition image input devices are coming to be widely used in products even in a reasonable price range. Similarly, a 5 million-pixel product has also put into practical use. This tendency of increasing the number of pixels that are being handled is said to continue still for the time being.
On the other hand, high-definition image output performance and cost reduction on products providing a hard copy such as that of laser printers, ink-jet printers, sublimated printers, as well as providing a soft copy of flat-panel display devices such as CRTs, LCDs (liquid crystal display devices), PDPs (plasma display device), and so forth are being achieved. Introducing highperformance, inexpensive image input/output products increases availability of high-definition images. Thus, in various scenarios, demand for high-definition images increases. In fact, development in technology relevant to communications networks, including personal computers and the Internet is actually accelerating this trend, especially, the spread speed of mobile apparatuses, such as cellular phones and notebook-type personal computers, is very high recently, and the opportunity to transmit or receive high definition images using the communications devices from all points which is increasing rapidly.
Accordingly, it should be inevitable that the demand for image compression/decompression technology to have highly-efficient performance and multi-functions, enabling easy handling of highly definition images, will increase. JPEG2000 has been developed so as to fulfill the demand by restoring high definition images from compressed image data at a high data compression rate. In JPEG2000, generally, a given image is divided into rectangular areas (tiles), and each tile is regarded as a basic unit when performing the data compression/decompression process. Therefore, compression/decompression operation is performed for every tile individually. In this case, when compression/decompression processing is performed with a high rate of compression, a problem occurs in that “the boundary of a tile” becomes noticeable in a decompressed restored image. In order to solve this problem, making a boundary overlap mutually with adjoining tiles has been proposed. However, according to the base line of JPEG2000, overlapping of adjacent tiles is not allowed.
According to another proposal, a lowpass filter may be applied around a tile boundary so as to prevent the tile boundary from becoming noticeable. Although this scheme may be effective to reduce a tile boundary distortion, another problem may occur in that an edge is inevitably faded near the tile boundary and a belt shaped zone of image quality degradation may occur there.
For example, according to an image coding apparatus and an image decoding apparatus disclosed by Japanese laid-open patent No. 11-285006, tile circumference data is obtained by extrapolation at a time of applying a wavelet transform, management information is created, and it is attached to code data. Then, the code data is decoded with reference to the management information at a time of performing the decoding operation. In this scheme, extra data should be attached to the code data.
According to a method of reducing block distortion at the time of decoding image data obtained through transform coding and a transformed image data decoding apparatus disclosed in Japanese patent No. 2940422, a lowpass filter is applied in an adaptive way in accordance with quantization degree in an adjacent block.
According to Japanese laid-open patent application No. 05-014735 (image-processing apparatus), a block boundary is determined, and, an even smoothing filter is applied to the determined block boundary.
The above-mentioned prior art may not necessarily be sufficient in effectively achieve a block boundary distortion.